Glycosylated hemoglobin standards

ABSTRACT

This invention encompasses a standard for determining glycosylated hemoglobin comprising a mixture of hemoglobin or methemoglobin and about 1% to 25% of the N-[5-nitrotropon-2-yl] hemoglobin or methemoglobin derivative wherein the allosteric binding site is blocked.

SUMMARY OF THE INVENTION

This invention encompasses standard reagents useful in determining glycosylated hemoglobin in the blood. A mixture of hemoglobin or methemoglobin and 1% to 25% of the N-[5-nitrotropon-2-yl] hemoglobin or methemoglobin derivative wherein the allosteric binding site of the hemoglobin or methemoglobin is blocked are suitable standards. Thus standards of the present invention mimic mixtures of glycosylated and nonglycosylated hemoglobin with respect to spectral properties associated with the R and T transformation states of hemoglobin, i.e., they are synthetic glycosylated hemoglobin standards. Typically standards containing 3%, 11% and 20% of the N-[5-nitrotropon-2-yl] derivative are incorporated into test kits.

BACKGROUND OF THE INVENTION

Hemoglobin exists in two allosteric forms. The T (taught) and the R (relaxed) form. These forms have different chemical and physical properties and the relative amounts of R and T hemoglobin can be determined by art recognized techniques such as ultraviolet, infrared, visible, nuclear magnetic resonance, and electron spin resonance spectroscopy. For example, Perutz, et al., Biochemistry, 17, No. 17, 3641 (1978) describes absorption spectra of hemoglobin derivatives, i.e., R→T transition as a function of ligand and inositol hexaphosphate binding. Circular dichroism and chemical reactivity are among other techniques for distinguishing R and T states of hemoglobin. The relative amount of R and T states can be determined by both end-point and kinetic techniques.

Elevated levels of glycosylated hemoglobin are known to be associated with diabetes mellitus. Glycosylated hemoglobin is present in nondiabetics at a level of about 5% of total hemoglobin, while diabetics have 2-4 times that amount (Science, 200 April 7, 1978). Glycosylated hemoglobin level provides an index of a patient's average blood glucose concentration over a long period of time. This index is not affected by short-term fluctuations in blood sugar (hour-to-hour) and, hence, gives a relatively precise reflection of the state of blood glucose control in diabetics.

Glycosylated hemoglobin is commonly referred to as HbA, or fast hemoglobin because it migrates faster on a chromatograph column and, indeed, is generally measured by chromatography of electrophoresis.

It has recently (U.S. Ser. No. 973,368 which has a common assignee to this application) been discovered that the percent of glycosylated hemoglobin in blood can be measured by monitoring the shift in the equilibrium populations of R and T allosteric forms of hemoglobins when the nonglycosylated hemoglobin is reacted with an allosteric site binding substance. This reaction causes a shift from the R to the T allosteric form in the nonglycosylated fraction portion of the hemoglobin. The glycosylated hemoglobin in the blood sample does not contribute to the shift in the equilibrium of the allosteric forms since glycosylation blocks the allosteric binding site. Thus, the higher the percentage of glycosylated hemoglobin in the blood sample, the smaller the shift between allosteric forms upon reacting the hemoglobins with an allosteric site binding substance. The earlier discovery takes advantage of the reactivity of the allosteric binding site which is accessible in nonglycosylated hemoglobin and the resulting shift in the equilibrium of allosteric forms of the glycosylated and nonglycosylated hemoglobin mixture resulting when an allosteric binding site substance is reacted with the nonglycosylated hemoglobin fraction.

A wide variety of compounds are known as effective allosteric effector site binding substances, commonly referred to as the organophosphate binding site of hemoglobin. These include organophosphates, sulfates, carboxylic acids represented by inositol hexaphosphate, J. Biol. Chem., 246, 7168 (1971); 2,3-diphosphoglycerate, Nature, 234, 174 (1971); adenosine triphosphate, Biochem. Biophys. Res. Comm., 26, 162 (1967); pyridoxal phosphate, Fed. Proc. Fed. Amer. Soc., Expl. Biol., 28, 604 (1969); inositol pentaphosphate, Can. J. Chem., 47, 63 (1969); 8-hydroxyl-1, 3,6-pyrenetrisulfonate, J. Biol. Chem., 246, 5832 (1971); 0-iodosodium benzoate, The Journal of Pharmacology and Experimental Therapeutics, 203, 72 (1977). Those skilled in the hemoglobin arts will recognize a wide variety of allosteric site binding substances. Inositol hexaphosphate is a preferred allosteric effector site binding substance.

It is generally desirable to lyse red blood cells to release hemoglobins. Common cationic (e.g., cetyl trimethyl ammonium bromide); anionic (e.g., sodium dodecylsulfate and sodium deoxycholate) and neutral (e.g., saponin and octyl phenosypoleythoxyethanol) detergents are useful in lysing red blood cells. Neutral detergents in the concentration range of about 0.025 to 0.5 volume percent are preferred. Mechanical rupture, for example, ultrasonication and hypotonic lysis, are also effective ways of releasing hemoglobin from red blood cells.

Binding of heme-binding ligands to heme iron generally shifts the equilibrium of allosteric hemoglobin isomers to the relaxed (R) form. Thus, when the heme-binding moiety of the hemoglobins in the test sample is coordinated with a heme-binding ligand larger shifts in the equilibrium populations of allosteric forms of hemoglobin are observed. This magnification in shift in equilibrium enhances accuracy and precision of glycosylated hemoglobin determination. This coordination of heme-binding ligand to shift equilibrium of allosteric isomers is applicable when the iron is in the Fe⁺² or the Fe⁺³ (methemoglobin) states.

Those skilled in the hemoglobin arts will recognize a wide variety of heme-binding ligands which bind to the iron of hemoglobin or methemoglobin.

For example, isocyanides such as alkyl isocyanides having 1-6 carbon atoms or phenyl isocyanides are particularly desirable heme-binding ligands for hemoglobin in the Fe⁺² state. Other suitable ligands are O₂ and NO.

It is generally preferred to have a single ligand bound to iron since this results in simpler measurements of the shift in allosteric forms. For example, oxyhemoglobin (glycosylated and nonglycosylated) is preferably deoxygenated by reaction with sodium dithionite or other well-known reducing agents to deoxyhemoglobin. The deoxyhemoglobin is reacted with alkylisocyanide such as n-butylisocyanide and as a result reaction with an allosteric effector site binding ligand provides a more definite shift in equilibrium of the allosteric forms permitting determination of glycosylated hemoglobin.

Hemoglobin is oxidized to methemoglobin by art recognized techniques, Antonini and Brunoni, Hemoglobin and Myoglobin in Their Reaction With Ligands, North Holland Publishing Co., Amsterdam (1971). Thus, potassium ferricyanide, sodium nitrite, aniline, and phenylhydrazine are convenient reagents for oxidizing hemoglobin to methemoglobin. Autooxidation in the presence of dyes such as methylene blue also oxidizes hemoglobin to methemoglobin.

Nonglycosylated methemoglobin is reactive with allosteric effector site binding substances described for nonglycosylated hemoglobin.

Those skilled in the hemoglobin arts will recognize a large variety of heme-binding ligands which bind with methemoglobin. These ligands include cyanate, thiocyanate, N-hydroxyacetamide, imidazole and derivatives thereof. Perutz, et al., Biochemistry, 17, 3640-3652 (1978).

Other common ligands are fluoride, azide, nitrite, cyanide, water, hydroxide ammonia, acetate and formate. Imidazole at about 0.1 M is a preferred heme-binding ligand for use with methemoglobin.

Typically, 1 ml of a reagent which is 0.1 M imidazole, 0.2 mM potassium ferricyanide, K₃ Fe(CN)₆, and 0.05% by volume triton X-100 (octyl phenoxypolyethoxyethanol) detergent in buffer at pH 6.8 is added to 10-20 μl of whole blood and the mixture is incubated for ten minutes.

The potassium ferricyanide oxidizes the hemoglobin to methemoglobin; the triton X-100 is a neutral detergent which lyses the cells to release hemoglobins; and the imidazole coordinates with the iron shifting equilibrium allosteric isomers to the (R) form.

The absorption spectrum of this mixture is recorded at 560 nm and 635 nm. Then 2 μl of a 0.1 M inositol hexaphosphate solution, pH 6.8 is added. The latter reagent reacts with the allosteric binding site of nonglycosylated hemoglobin and shifts equilibrium of the allosteric isomers to the (T) target form. The absorption spectrum at 560 nm and 635 nm is measured again. Glycosylated hemoglobin concentration is reflected by a decrease in 560 nm absorption and increased in the 635 nm absorption.

Reagent A: 0.1M imidazole, .2 mM K₃ Fe(CN)₆, 0.05% v/v triton X-100 (octyl phenoxypolyethoxyethanol detergent), in water, pH 6.8

Reagent B: 0.1 M inositol hexaphospate (IHP), in water, pH 6.8

To 1.0 ml of Reagent A at 25° C. add 10-20 μl whole blood, incubate 10 minutes to allow for cell lysis and oxidation of hemoglobin to methemoglobin. Record visible spectrum, 450 nm to 700 nm, specifically monitoring absorbance at 560 nm and 635 nm. Then add 2 μl Reagent B to the reaction mixture. Record another spectrum as before.

Standards prepared by methods of this invention are used to determine glycosylated hemoglobin present.

    ______________________________________                                          RESULTS                                                                       Standard Curve                                                                                               Normalized                                               No IHP    + IHP       Difference                                        % Glycosylated Hb                                                                        560nm A                                                                                635nm  A                                                                               560nm A                                                                               635nm A                                                                             ##STR1##                                ______________________________________                                          0%       0.664   0.089   0.592 0.123 0.184                                     5%       0.654   0.086   0.588 0.120 0.176                                    10%       0.657   0.089   0.593 0.121 0.169                                    15%       0.658   0.090   0.596 0.118 0.158                                    20%       0.663   0.095   0.609 0.123 0.144                                    25%       0.651   0.091   0.600 0.117 0.138                                    50%       0.645   0.098   0.611 0.113 0.090                                    100%      0.717   0.123   0.715 0.128 0.012                                    ______________________________________                                          Calculations: Δ = A.sup.560nm - A.sup.635nm                              ##STR2##                                                                       -                                                                        

DETAILED DESCRIPTION OF THE INVENTION

Hemoglobin in red blood cells or in solution is reacted with an excess of 2-loweralkoxy-5-nitrotropone wherein the lower alkoxy group contains 1 to 4 carbon atoms and the resulting hemoglobin derivative isolated and used or is oxided to methemoglobin. The reaction is as follows: ##STR3##

R represents loweralkyl having 1-4 carbon atoms and P represents protein of hemoglobin or methemoglobin.

In the case of initiating the process with red blood cells, the cells are reacted with, for example, 2-methoxy-5-nitrotropone lysed, and the hemoglobin is oxidized to methemoglobin. In this process several N-[5-nitrotropon-2-yl] groups are bound to the hemoglobin and in particular the allosteric binding site is blocked by one of these tropone groups. Alternatively, the process is initiated with a hemoglobin solution which is reacted with an excess of 2-methoxy-5-nitrotropone and the derivatized hemoglobin is either isolated or oxidized to methemoglobin. It is generally desirable to use the methemoglobin derivative because of its stability.

However, it should be pointed out that red blood cells wherein the hemoglobin has been derivatized to block the allosteric binding site can be suspended in 10%-30% glycerol/saline and used as a standard in that form.

Although the reaction of 2-methoxy-5-nitrotropone with hemoglobin is known Eur. J. Biochem., 91, 285-289 (1978), the N-[5-nitrotropon-2-yl] derivative of methemoglobin is not known.

Standard mixtures or solutions of the N-[5-nitrotropon-2-yl] methemoglobin derivative wherein the allosteric binding site of the methemoglobin is blocked is preferred. Mixtures of methemoglobin 1% to 25% of the methemoglobin derivative is aqueous solution are preferred.

In a particularly preferred embodiment for use in test kits, mixtures of methemoglobin and 3%, 11% and 20% of the N-[5-nitrotropon-2-yl] methemoglobin derivative wherein the allosteric binding site is blocked in a 5-15 millimolar solution in phosphate buffer pH 6.8 or water is used.

The mixture may also be used as a lyophilized standard mixture.

The standard reagents of this invention are used in place of mixtures of hemoglobin and glycosylated hemoglobin.

The hereinafter set out examples are intended to illustrate the invention and are not intended to limit the invention in spirit or scope.

EXAMPLE 1

One-tenth liter of whole blood is stirred and centrifuged at 1800-2000 xg for thirty minutes at 25° C.

The supernatant is aspirated and the cells are washed with normal saline and then suspended in 250 ml of phosphate buffered saline pH 7.2. The suspension is added to 4.5 liters of phosphate buffered saline, pH 7.2 To this suspension is added 0.6 grams of 2-methoxy-5-nitrotropone and the resulting mixture is stirred at 25° C. for 6-7 hours. The cell suspension is centrifuged at 1800×2000 xy for 30 minutes and the supernatant liquid is decanted. The cells are washed by resuspending in 250 ml of normal saline, centrifuged at 1800-2000 xg for 30 minutes, and resuspension. This latter process is repeated three times. The cells are lysed by suspension in water for one hour at 25° C. overnight at 4° C. This suspension of lysed cells is centrifuged 15,000-17,000 xg for 30 minutes and the supernatant retained and stored at 2° to 8° C. The hemoglobin derivative in the supernatant is oxidized to methemoglobin derivative by treatment with 0.4 grams of potassium ferricyanide. The 2-methoxy-5-nitrotropone derivative of methemoglobin is isolated on a mixed bed ion exchange resin and 1/20 volume of 1 molar potassium phosphate, pH 6.2, is added. This solution is concentrated by dialysis on a Amicon hollow fiber dialyzer/concentrator apparatus.

The 2-methoxy-5-nitrotropone derivative of methemoglobin is mixed with pure methemoglobin to obtain standards having from 1% to 25% of the 2-methoxy-5-nitrotropone derivative of hemoglobin. Standards having a concentration of 5-15 millimolar protein in phosphate buffer pH 6.8 wherein the methemoglobin derivative is 3%, 11% or 20% are preferred.

EXAMPLE 2

The 2-methoxy-5-nitrotropone derivative of hemoglobin is obtained according to the procedures in Example 1 except oxidation with potassium ferricyanide is omitted.

EXAMPLE 3

Derivative produced in Example 1 and 2 are produced by initiating the process with a hemoglobin solution in place of red blood cells or first lysing the red blood cells prior to derivatization.

EXAMPLE 4

Following the procedure of Example 1 wherein the hemoglobin in the red blood cells is derivatized with 2-methoxy-5-nitrotropone and the red blood cells are not lysed but are washed and stored in 10%-30% glycerol/saline to provide a stable standard. Thus an aqueous suspension of red blood cells wherein 1% to 25% of the red blood cells have hemoglobin derivatized to form the N-[5-nitrotropon-2-yl] hemoglobin wherein a N-[5-nitrotropon-2-yl] group blocks the allosteric binding site of hemoglobin. Typically suspensions of 3%, 11%, and 20% in glycerol in saline solution are incorporated into test kits. 

I claim:
 1. A standard for determining glycosylated hemoglobin comprising a mixture of hemoglobin and 1% to 25% of the the N-[5-nitrotropon-2-yl] hemoglobin wherein the allosteric binding site of the hemoglobin is blocked.
 2. A standard, according to claim 1, wherein the mixture is in the range of 5 to 15 millimolar concentration in aqueous solution.
 3. A standard for determining glycosylated hemoglobin comprising a mixture of methemoglobinand 1% to 25% of the N-[5-nitrotropon-2-yl] methemoglobin wherein the allosteric binding site of the methemoglobin is blocked.
 4. A standard, according to claim 3, wherein the mixture is in the range of 5 to 15 millimolar concentration in aqueous solution.
 5. A standard for determining glycosylated hemoglobin comprising a suspension of red blood cells wherein 1% to 25% of red blood cells have hemoglobin derivatized to form N-[5-nitrotropon-2-yl] hemoglobin wherein a N-[5-nitrotropon-2-yl] group blocks the allosteric binding site of the hemoglobin.
 6. A standard, according to claim 5, wherein the suspension is a 10%-30% glycerol in saline solution. 